Visible-Light Driven Photocatalytic Degradation of Organic Dyes

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Article

Visible-Light Driven Photocatalytic Degradation of Organic Dyes over Ordered Mesoporous CdZn S Materials x

1-x

Yoon Yun Lee, Jong Hun Moon, Yun Seok Choi, Gwi Ok Park, Mingshi Jin, Longyi Jin, Donghao Li, Jin Yong Lee, Seung Uk Son, and Ji Man Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00038 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 20, 2017

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The Journal of Physical Chemistry

Visible-Light Driven Photocatalytic Degradation of Organic Dyes over Ordered Mesoporous CdxZn1-xS Materials Yoon Yun Lee,† Jong Hun Moon,‡ Yun Seok Choi,‡ Gwi Ok Park,‡ Mingshi Jin,§ Long Yi Jin,§ Donghao Li,§ Jin Yong Lee,*,‡ Seung Uk Son,*,‡ and Ji Man Kim*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 16419,

Republic of Korea ‡

Department of Chemistry, Sungkyunkwan University, Suwon, 16419, Republic of Korea

§

Key Laboratory of Natural Resource of the Changbai Mountain and Functional Molecular,

Ministry of Education, and Department of Chemistry, Yanbian University, Park Road 977, Yanji, 133002, China

ACS Paragon Plus Environment

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ABSTRACT: Highly ordered mesoporous CdxZn1-xS materials were obtained via a simple nanoreplication method using a mesoporous silica template with a 3-D bicontinuous cubic Ia3d mesostructure. Combined analyses using X-ray diffraction, N2 sorption, electron microscopy, and diffuse reflectance UV-visible spectroscopy revealed that the ordered mesoporous ternary compound semiconductor materials exhibited well-developed crystalline frameworks, high surface areas of 80 – 120 m2g-1, uniform mesopore sizes of about 20 nm, ordered arrangement of mesopores, and outstanding visible light absorption properties. Photocatalytic activities were investigated by degradation of methylene blue and rhodamine B under visible light over the mesoporous CdxZn1-xS materials. Due to the high surface area and outstanding light absorption properties, the ordered mesoporous CdxZn1-xS exhibited excellent photocatalytic performances for the degradation of methylene blue and rhodamine B. This study indicates a potential application of the mesoporous compound semiconductors in the efficient visible-light-driven photolysis of organics that may cause environmental pollution.


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1. INTRODUCTION Since the discovery of photocatalytic water splitting under ultraviolet (UV) light illumination by using TiO2, the syntheses and photocatalytic applications of TiO2 with high UV absorption property and reasonable stability have attracted much attention for the purpose of environmental remediation such as degradation of organic pollutants, water purification, and decomposition of carbonic acid gases.1-4 In photocatalysis, anatase TiO2 is more efficient than rutile phase due to a more open crystal structure that is good for diffusion of reactants and light. However, the relatively poor stability of anatase TiO2 renders crystal structure transformation into the less efficient rutile phase upon heat treatment above 500 ˚C.5 Although TiO2 is the most widely used material in heterogeneous photocatalysis because of its advantages (cheap, chemically inert, etc.), some drawbacks do exist which undermine its photo-efficiency in practical applications. A wide band gap (e.g. 3.2 eV for anatase TiO2), necessitates UV light irradiation for photo-excitation to generate electron–hole pairs.6 The UV light possesses only a small portion (about 5%) of the entire solar spectrum, and the rest is predominantly visible light. Any observable shift in the photo-response of TiO2 from UV to visible light regime would enhance much of its photocatalytic activities.7 Researchers have tried to extend the photo-response of TiO2 into the visible light region, through the doping with transition metals or non-metal ions.7-8 However, although the transition metal doping of TiO2 decreases the photo-excitation energy to some extent, the metal ions also act as the recombination sites for the electron–hole pairs, thus to undermine the overall efficiency of the photocatalyst.9 Moreover, elemental doping to the TiO2 crystal lattice requires quite complex experimental procedures and involves high temperature treatment. These processes will likely cause the anatase phase TiO2 to undergo phase-transition into the less active rutile phase.10


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Development of alternative semiconductors with capacities to absorb visible light (the largest portion, 44 ~ 47% of the solar spectrum reaching the earth’s surface) is required to enhance the photocatalytic efficiency. Therefore, the exploration of photo-chemically active semiconductors, especially with high photocatalytic activity under visible light irradiation, is one of the most challenging topics in the global environmental and energy problems.6, 11-14 The nano-structured compound semiconductors with reasonable band gap energies can be regarded as a good candidates for photocatalytic decomposition of organic pollutants under visible light illumination. In particular, some metal chalcogenides have outstanding visible light absorption properties with band gap energies that correspond well to the solar spectrum, and with conduction band edges that are more negative than the H+/H2 redox potential. Owing to their excellent optical and physicochemical properties, there have been great interests in the design and application of nanostructured metal chalcogenides.15 Among the various metal chalcogenides, CdS and ZnS are most widely regarded as suitable materials for photocatalysis, due to their unique optical properties and suitable band positions for the photocatalytic degradation of organics in aqueous solution (the lowest conduction band potential is more negative than the redox potential of H+/H2; the highest valence band potential is more positive than the redox potential of O2/H2O). Various nanostructures of CdS and ZnS have been investigated as a photocatalyst for H2 evolution reaction (HER), alcohol oxidation to form aldehyde or ketones, and decomposition of organic dyes or pollutants under visible light illumination.16-20 Typically, the mesoporous materials show higher photocatalytic activity than other nano-structured materials such as nanoparticles, nanowires, nanorods, and so on, due to some advantages: (1) can interact with ions and molecules not only at its internal surfaces but also all over the bulk of the material, (2) high internal surface areas provide more active sites, and (3) proper mesopore sizes offer ease penetration and diffusion of reactant


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molecules. For example, the mesoporous CdS with 3-D networks exhibits higher conversion efficiency in conversion of 1-phenylethanol to acetophenone compared with CdS nanocrystal.21-22 In this study, the band gap energy-tuned mesoporous CdxZn1-xS materials (from 2.43 to 3.64 eV) were successfully prepared by a nano-replication method using an ordered mesoporous silica KIT6 (bicontinuous cubic Ia3d structure) as a hard-template. The band gap energy of the replicated mesoporous CdxZn1-xS was chemically controlled by control of the molar composition of metals (Cd/Zn). Because the band gap energies of the mesoporous CdxZn1-xS are suitable for absorbing visible light, it can be used as visible-light-driven photocatalysts for decomposition of organic pollutants. Consequently, the band gap energy dependent photocatalytic activities of the mesoporous CdxZn1-xS for the degradation of organic dyes, methylene blue and rhodamine B, were measured under visible light irradiation (λ ≥ 400 nm). 2. EXPERIMENTAL SECTION 2.1. Synthesis of 3-D cubic Ia3d mesoporous silica KIT-6. Ordered mesoporous silica KIT-6 was synthesized from published procedure, and all the chemicals were used as received.23-24 Firstly, 30.0 g of Pluronic® P-123 block copolymers (EO20PO70EO20, Aldrich, Mw = 5,800) were dissolved in 30.0 g of n-butanol (C4H9OH, Aldrich, > 99.0 %), and 1085 g of distilled water added to mixture of them at room temperature followed by aging for 6 h to achieve complete dissolution of block copolymers. The clear polymer solution was kept in 35 °C water bath for 6 h in order to reach thermal equilibrium, then, 59 g of hydrochloric acid (HCl, Samchun Chemical, 35 ~ 37 %) and 64.5 g of tetraethyl orthosilicate (Si(OCH2CH3)4, Samchun Chemical, 98 %) were added into this solution. The reaction mixture was vigorously stirred for 24 h at 35 °C, and subsequently kept for another 24 h in an oven at 100 °C under static conditions. The white precipitates were filtered, dried and washed with distilled water, and some of the polymers were removed by ethanol


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extraction process. To remove the all Pluronic® P-123 block copolymers completely, the product was calcined in air at 550 °C for 3 h. 2.2 Preparation mesoporous CdxZn1-xS photocatalyst. The ordered mesoporous CdxZn1-xS materials were obtained by published method.25 The reagent for synthesizing the ordered mesoporous CdxZn1-xS, cadmium sulfate hydrate (3CdSO4·8H2O, Aldrich, 99.999 % trace metal basis) and zinc sulfate hydrate (ZnSO4·7H2O, Aldrich, 99.999 %, trace metal basis) were used as received. In order to synthesize the ordered mesoporous CdS or ZnS, 1.06 g of 3CdSO 4·8H2O or 1.77 g of ZnSO4·7H2O were dissolved in 1.5 g of doubly distilled water, respectively. A solution containing the precursors was then infiltrated into mesopore of KIT-6 silica template (1.5 g) via incipient-wetness method. After drying at 80 °C oven for 12 h, the precursor/silica template composite was heated to 500 °C for 3 h under H2 atmosphere for reductive sulfurization of precursor reagents. Subsequently, the H2 flow was changed with N2 flow, and the furnace was cooled down to room temperature. The silica template was almost completely removed by using 2 M NaOH aqueous solution three times. The mesoporous CdxZn1-xS materials were prepared by same synthetic procedures described above with controlled molar composition between the Cd and Zn precursors. 2.3 Characterization. Powder X-ray diffraction (XRD) patterns were obtained in reflection mode using a Rigaku Ultima IV instrument equipped with Cu Kα (λ = 1.54 Å) radiation operating at 40 kV and 30 mA. Scanning electron microscopy (SEM) images were obtained using a Hitachi UHR S 5500 FE-SEM operating at 30 kV, and transmission electron microscopy (TEM) images were taken using a JEOL JEM 3010 instrument at an accelerating voltage of 300 kV. An N2-sorption isotherms were measured with a Micromeritics Tristar 3000 at liquid N2 temperature, and all of


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the samples were completely dried under vacuum at 100 °C for 24 h before the measurement. The diffuse reflectance UV-visible spectra were obtained with a Shimadzu UV-3600 spectrometer. 2.4 Photocatalytic degradation of organic dyes. The photocatalytic activities of the ordered mesoporous CdxZn1-xS materials were evaluated by photocatalytic degradation of methylene blue (MB, C16H18N3SCl, Fluka, for microscopy) and rhodamine B (RhB, C28H31ClN2O3, Sigma, ≥ 95 %), in an aqueous solution. A 250 mL Pyrex® beaker and 200 W Xe lamp equipped with a cutoff filter (λ ≥ 420 nm) were used as a reaction vessel and a light source, respectively. 50 mg of the photocatalyst, ordered mesoporous CdxZn1-xS materials, was suspended in a 100 mL aqueous solution of 2 × 10-5 M methylene blue or rhodamine B. Before the light irradiation, a suspension containing organic dyes and photocatalyst particles was sonicated for 10 min and vigorously stirred for 30 min in the dark to allow dispersion of photocatalyst particles in suspension and sorption equilibrium, respectively. To retain reaction temperature and absorb infra-red region during the photocatalysis, cooling water circulation system was equipped outer the reaction vessel as shown in Scheme 1. In the present work, the concentrations of methylene blue, rhodamine B, and their degradation products were analysed by checking the absorbance at 660 nm and 553 nm, respectively, by using UV-visible absorption spectrometer. At given time intervals, 2 mL aliquots were extracted and filtered by cellulose acetate syringe filter (pore size = 0.20 μm, 13 mm in diameter, Advantec, 13CP020AS) for removal of photocatalyst particles. 3. RESULTS AND DISCUSSION 3.1. Characterization of mesoporous CdxZn1-xS photocatalyst. Low- and wide-angle powder XRD patterns of the mesoporous CdxZn1-xS materials with various chemical composition (x = 0 – 1) were represented in Fig. 1. As depicted in Fig. 1A, all the low-angle XRD patterns of the mesoporous CdxZn1-xS materials are different from that of KIT-6 silica template (see Fig. S1 (a)).


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A new peak appeared in the lower angle region than 2θ of 0.5 °, which corresponds to (110) reflection plane, and appearance of this new peak represents the mesostructure transition from cubic Ia3d to tetragonal I41/a or lower space group after the silica removal process. This phenomenon occurs due to the specific pore system of Ia3d mesostructured KIT-6 silica template. The mesoscopic phase transformation could be observed in the replication of other mesoporous materials via nano-replication methods using Ia3d structured silica such as KIT-6 and MCM-48.2630

When only one of the two chiral pore channels is incorporated with precursors, large pores

(typically > 10 nm) generated after the silica template etching process due to the two walls (wallthickness of KIT-6 silica: 3.75 nm) and one mesopore (7.3 nm) of silica template are converted into mesopore of the resulting mesoporous CdxZn1-xS material. The crystallinity of the mesoporous CdxZn1-xS was characterized by a wide-angle XRD technique (Fig. 1B). The mesoporous CdS (Fig. 1B (a)) and ZnS (Fig. 1B (g)) show wurtzite hexagonal P63mc and zinc blende cubic F-43m, respectively. Because the mesoporous CdS has sharper diffraction peaks than ZnS, crystallite size of CdS would be larger than ZnS. The calculated crystallite sizes of the mesoporous CdS by Scherer’s formula of about 13 nm is larger than mesopore size of silica template. It is expected that crystallization process of CdS occurred at outside of silica mesopores, therefore, mesostructures collapsed after silica template removal and there are no specific diffraction peaks in low-angle XRD pattern in Fig. 1A (a). In contrast, the mesoporous ZnS has a crystallite size of about 5 nm, smaller than mesopore size of silica template, so it would be crystallized in silica pores, and formed ordered mesoporous network. As a result, the replicated mesoporous ZnS shows several diffraction peaks in a high-angle region (Fig. 1B (g)). The crystal structure of Cd rich (x = 0.5 – 0.9) materials are similar to that of wurtzite CdS, and Zn rich (x = 0.1 – 0.3) materials are similar to that of zinc blende ZnS. Thus, the diffraction


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peaks of the mesoporous CdxZn1-xS materials are gradually shifted with the changes of Cd/Zn ratio, owing to the atomic size difference of them. Fig. 2A shows N2 adsorption-desorption isotherms of the mesoporous CdxZn1-xS materials. All of the mesoporous CdxZn1-xS materials exhibit type-IV isotherms with H1 hysteresis loop in the range of p/p0 = 0.8 – 1.0, and high surface area of 81.2 – 131.8 m2g-1 and large total pore volume of 0.32 – 0.93 cm3g-1. The pore size distributions achieved from the adsorption branches of about 20 nm are very similar to each other despite of their chemical composition differences (Fig. 2B). A pore size of 20 nm is much larger than the wall-thickness of KIT-6 silica template, and it is well known that the large pores generated when the microporous linker channel between the two chiral pore systems of KIT-6 is broken, resulting single gyroid structures with mesostructure transition from cubic Ia3d of KIT-6 silica to tetragonal I41/a or lower symmetry of replica, which matched with low-angle XRD patterns. The details of physical properties for the mesoporous CdxZn1-xS materials are shown in Table 1. The overall particle morphologies and several micrometers (μm) of particle sizes of the KIT-6 silica template and replicated mesoporous CdxZn1-xS materials are similar to each other as depicted in Fig. S1 and Fig. 3 (a – c). Thus, as confirmed by the HR-SEM images in the insets of Fig. 3 (a – c), all the mesoporous CdxZn1-xS materials exhibited highly ordered mesostructures over the whole particles. The mesoporous CdS shows disordered mesostructures (Fig. 3 (d)), and this is good agreement with low-angle XRD pattern in Fig. 1A (a). The HR-TEM image of the mesoporous CdS in Fig. 3 (d) inset indicates the (002) plane of crystallite, and it also supports the developed crystallinity which was suggested by the wide-angle XRD pattern in Fig. 1B (a). Slightly different from the mesoporous CdS, the mesoporous Cd0.5Zn0.5S and ZnS show ordered mesostructures and uniform pre sizes as depicted in Fig. 3 (e, f). The HR-TEM images of the


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mesoporous Cd0.5Zn0.5S, and ZnS in Fig. 3 (e, f) insets are (103) and (311) crystal planes, respectively. The visible light sensitivity of the mesoporous CdxZn1-xS materials was characterized by diffuse reflectance UV-visible spectroscopy. As shown in Fig. 4A, the light absorption edges of the mesoporous CdxZn1-xS materials are gradually shifted to a shorter wavelength region with Cd composition x as decreases from 1.0 (CdS) to 0.0 (ZnS). Fig. 4B represents the Kubelka-Munk plots for determining the band gap energy of the mesoporous CdxZn1-xS materials. The estimated band gap of the mesoporous CdS (2.43 eV) and ZnS (3.64 eV) are slightly larger than those of bulk CdS (wurtzite 2.42 eV) and ZnS (zinc blende 3.50 eV). The band gap expansions of the mesoporous CdS and ZnS are related to the quantum confinement effects, due to their nano-sized frameworks. The band gap energies of the mixed mesoporous CdxZn1-xS materials are almost linearly increased from 2.47 to 3.47 eV with decreases of Cd composition x as goes from 0.9 to 0.1, and detailed values are presented in Table 1. 3.2 Photocatalytic degradation of organic dyes. In the presence of air or oxygen, the light illuminated semiconductor photocatalysts are capable of destroying many organics. The activation of semiconducting materials by light energy (hv) generates electron (e–) and hole (h+) pairs which are reductant and oxidant, respectively. In the degradation of organics, the hydroxyl radial (·OH) which comes from the oxidation adsorbed water or adsorbed hydroxyl (–OH), is the primary oxidant; and the presence of oxygen could prevent the recombination of electron-hole pairs. For a complete photocatalytic degradation of methylene blue (MB) and rhodamine B (RhB), the final products of the reaction among others are CO2 and H2O. C16H18N3SCl → CO2 + H2O + NH4+ + NO3– + SO42– + Cl–

(MB)

C28H31ClN2O3 → CO2 + H2O + NO3– + Cl–

(RhB)


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In order to determining the concentration of organic dyes at given time after photocatalysis initiated from the UV-visible absorption spectrums, standard curves for methylene blue and rhodamine B (relationship between the absorbance and concentration) were prepared (see Fig. S2). Fig. 5 shows photocatalytic degradation of methylene blue and rhodamine B in terms of relative concentration changes as a function of the light irradiation time in the presence of mesoporous binary compound semiconductors, CdS and ZnS, respectively, and detailed UV-visible spectrums are presented in Fig. S3. Due to the higher surface area and larger pore volume of mesoporous ZnS (123.7 m2g-1, 0.82 cm3g-1) than CdS (81.2 m2g-1, 0.38 cm3g-1), adsorption amounts of organic dyes on the surface of photocatalyst particles also larger in the case of mesoporous ZnS than CdS. During the sorption equilibrium process conducted for 30 min in the dark, only 2.1 % and 5.7 % of initial concentration of methylene blue and rhodamine B were adsorbed by mesoporous CdS (Fig. 6A (a) and 6B (a)), while mesoporous ZnS adsorbed 10.3 % and 13.4 % of dyes (Fig. 5A (b) and 5B (b)), respectively. As a result, the initial relative concentration diminutions of the mesoporous ZnS were larger than those of mesoporous CdS. The absorbance of organic dyes gradually diminished as the exposure time increased, which is more pronounced in the case of using mesoporous CdS as a photocatalyst. Therefore, the concentration changes of methylene blue and rhodamine B in the presence of mesoporous CdS particles is greater than in the presence of mesoporous ZnS so that for CdS photocatalyst, the concentration of methylene blue decreased by half after 30 min, and 99.1 % of methylene blue and 99.7 % of rhodamine B decomposed after 90 min while for the mesoporous ZnS, only 8.4 % of methylene blue and 10.5 % of rhodamine B were decomposed after 90 min. To show the visiblelight-driven photocatalytic activity, a small band gap is required for a photocatalyst material. Another important criterion for the photocatalytic degradation of organic compounds is that the


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redox potential of the H2O/·OH (E° = – 2.8 V) couple lies within the valence and conduction bands of the semiconducting photocatalysts. The different photocatalytic activities of the mesoporous CdS and ZnS might be related to their band gap energies and band alignment with a redox potential of H2O and O2. The larger band energy of the mesoporous ZnS (3.64 eV) gives a poor absorption of visible light range, although its conduction band bottom potential is sufficiently negative than the redox potential of H+/H2O and O2/O2–. In contrast, mesoporous CdS has small band gap energy of 2.43 eV which good for the visible light absorption and shows reasonable photocatalytic activity for degradation of organic dyes in aqueous media containing oxygen. In order to increase overall photocatalytic activities, mesoporous compound semiconductors with controlled band gap energies were prepared by controlled chemical composition of nanosized frameworks. For comparison, the concentration changes of methylene blue and rhodamine B in the presence of commercial TiO2 particle (Degussa P25) under exposure to the visible were measured and compared with those determined in the presence of mesoporous CdxZn1-xS materials as shown in Fig. 6A and 6C. Because of the wide band gap energy, P25 (TiO 2) particle could not absorb wavelength range of irradiated light (λ ≥ 420 nm) and show no photocatalytic activity as depicted in Fig. 6A (a) and 6C (a). Apparently, the photocatalytic activity of the mesoporous CdxZn1-xS materials strongly affected by their chemical composition. The Cd rich materials (0.7 ≤ x ≤ 1.0) almost perfectly decompose methylene blue and rhodamine B molecules after the light irradiation time of 90 min as seen in Fig. 6A (b – d) and 6B (b – d). The mesoporous Cd0.7Zn0.3S material, in particular, shows the best photocatalytic activity in both cases. In the case of mesoporous Cd0.5Zn0.5S, only 60 % of methylene blue and 56 % of rhodamine B were degraded after 90 min (Fig. 6A (e) and 6C (e)), and the photocatalytic activity was decreased with increasing Zn contents from 0.7 to 1.0 as depicted in Fig. 6A (f – h) and 6C (f – h). Detailed UV-visible


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absorption spectral changes methylene blue and rhodamine B by using P25 and the mesoporous CdxZn1-xS materials are presented in Fig. S4 and S5, respectively. Fig. 6B and 6D represent the linear plots of ln (C/C0) for the photo-degradation of methylene blue and rhodamine B with the mesoporous CdxZn1-xS photocatalyst under the visible light irradiation after 90 min, respectively. The slope of the plots which represents the apparent rate constant (kapp) of photocatalysis were calculated by ln(C/C0) = – kapp·t and listed in Table 2. It could be seen from Table 2 that the apparent reaction rate of the mesoporous CdxZn1-xS in the degradation of organic dyes increases with decreasing Cd ratio and reaches its maximum at an x = 0.7. The maximum apparent rate constants for methylene blue and rhodamine B are 7.88 × 10 -2 min-1 and 9.53 × 10-2 min-1, respectively. For the separate photocatalytic activities of the mesoporous CdxZn1-xS photocatalysts, the difference of the band gap energy would be the dominating reason that results in the different absorption properties in the visible light range.6 With the decline of Cd composition x in mesoporous CdxZn1-xS materials, the position of valence band edge hardly changes, whereas the conduction band edge becomes more negative, which lead to the noticeable blue-shift of the band gap energy of them (Fig. 7). The widened band gap energy of the mesoporous CdxZn1-xS results in reduced absorption of the visible light, which further leads to a decrease in photo-chemically generated electron–hole pairs. As a result, photocatalytic activities could be decreases. In contrast, the more negative conduction band edge of the photocatalyst is advantageous for the photoreduction, which might also affect the redox processes. The manipulation of the composition of the mesoporous CdxZn1-xS materials offers a powerful role in producing a photocatalyst with controlled activities.


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As discussed, there are two important reasons for the visible-light-driven photocatalytic degradation of organic dyes: 1) absorption capacity of the visible light, and 2) position of conduction band edge of the photocatalysts. The photocatalyst with smaller band gap energy, i.e. Cd rich phases in the mesoporous CdxZn1-xS, is better photocatalyst in the viewpoint of the visible light absorption ability. Thus, the conduction band edge position of photocatalyst mainly influences on electron transfer rate from the catalyst particle to water molecule. The energy difference between the conduction band edge and redox potential of oxygen in the water, i.e. ΔG in Fig. 8, strongly effects on the photo-reduction reaction which produce superoxide (O2–) or peroxides (O22–) anions, and Zn-rich materials are better photocatalysts in perspective of this point. Of course, hole transfer from the valence band of the photocatalyst to water to produce hydroxyl radical (·OH) also effects on the photocatalytic efficiency, however, the valence band positions of the mesoporous CdxZn1-xS are quite similar to each other in spite of their chemical composition difference of the frameworks. So, we could expect that the electron transfer rate between the photocatalyst particle and oxygen is the main factor for determining the decomposition efficiency. In practice, two factors competitively effect on the photocatalytic activities of the mesoporous CdxZn1-xS, and optimized compositional value (x = 0.7 in this case) for increasing activity exists between the two factors. In addition, the photo-decomposition efficiency of rhodamine B is higher than that of methylene blue despite of chemical composition difference of the mesoporous CdxZn1-xS photocatalysts. The electron transfer phenomenon from the conduction band of the photocatalyst to LUMO level of organic can reduce the photocatalytic efficiency because this electron quenched out in organic molecules by the relaxation processes. As shown in Fig. 8, this unfavourable electron transfer phenomenon more quickly occurred in methylene blue than rhodamine B, because of the LUMO


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level of methylene blue than the conduction band of the mesoporous CdxZn1-xS materials. However, this unfavourable electron transfer could be significantly suppressed in rhodamine B due to its high LUMO level energy. The photocatalytic activities of nano-structured TiO2, CdS, ZnS and CdxZn1-xS are summarized in Table 3. The 10 nm TiO2 nanoparticles completely decomposed within 2 h under UV-A (wavelength from 310 to 400 nm) irradiation, whereas mesoporous TiO2 degrade only 96 % under UV lamp of methylene.31-32 The 96 % of rhodamine B dye was degraded by commercial TiO2 (Degussa P25) photocatalyst after 2 h UV irradiation.33 However, due to the wide band gap energy, mesoporous TiO2 shows no photocatalytic activity under visible light (> 400 nm) irradiation despite of its high surface area (> 100 m2/g).34 In order to overcome the limitation of TiO2-based photocatalyst, semiconducting materials that can absorb visible light range such as CdS, ZnS and so on were also investigated as a photocatalyst. A mixture of CdS and ZnS nanoparticles (20 : 80) decomposed about 85 % of initial concentration of methylene blue after 6 h under strong visible light irradiation.35 Mesoporous ZnxCd1-xS nanoparticles prepared by W. Wang et al. shows the best photocatalytic activity for degradation of rhodamine B under visible light irradiation (100 % degraded before 15 min irradiation time).6 The mesoporous CdxZn1-xS microparticles which prepared in this work also perfectly removed methylene blue and rhodamine B within 70 min. However, the amount of photocatalyst used in this work was half of study by W. Wang et al., and the concentration of dye solution was double of them. We believe that this study will reveal the prospect of large-scale photocatalytic degradation of organic pollutant in aqueous solution. 4. CONCLUSION Highly ordered mesoporous CdxZn1-xS materials exhibiting high surface areas, large pore volumes, uniform pore size, developed crystallinity and micrometer-sized particles with compositional


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homogeneity were successfully obtained via a nano-replication method using a 3-D cubic Ia3d mesoporous silica KIT-6 as a hard-template. The optical property and band gap energy profile of the replicated template-free mesoporous CdxZn1-xS materials were easily controlled by manipulating the chemical composition of the frameworks. Diffuse reflectance UV-visible spectroscopy studies indicate that the band gap energies of the mesoporous CdxZn1-xS materials widened with decreasing the amounts of Cd. We investigated the mesoporous Cd xZn1-xS material as a visible-light-driven photocatalyst for the degradation of methylene blue and rhodamine B. Under visible light illumination, the mesoporous CdxZn1-xS materials showed compositiondependent photocatalytic activities for the decomposition of organic dyes. The photocatalytic efficiency of the mesoporous CdxZn1-xS increases evidently with a decrease in the Cd contents from 1.0 to 0.7, and reduced with the further decrease of Cd composition from 0.7 to 0.0. This finding presents new possibilities for novel chalcogenide semiconductors as the visible-light sensitive catalysts in photocatalytic applications, especially in the field of environmental remediation. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website at DOI:. X-ray diffraction patterns, N2 adsorption-desorption isotherms and TEM images of the KIT-6 silica template. UV-visible absorption spectrums of methylene blue and rhodamine B. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (J. Y. Lee); [email protected] (S. U. Son); [email protected] (J. M. Kim). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Research Council of Science and Technology (NST) through Degree and Research Center (DRC) Program (2014) (No. DRC-14-03-KRICT). REFERENCES (1) Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 2008, 20, 35-54. (2) Zhang, H.; Chen, G.; Bahnemann, D. W. Photoelectrocatalytic Materials for Environmental Applications. J, Mater, Chem, 2009, 19, 5089-5121. (3) Tong, H.; Ouyang, S.; Bi, Y.; Umezawa, N.; Oshikiri, M.; Ye, J. Nano-Photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229-251. (4) Ibhadon, A.; Fitzpatrick, P. Heterogeneous Photocatalysis: Recent Advances and Applications. Catalysts 2013, 3, 189. (5) Hanaor, D. A. H.; Sorrell, C. C. Review of the Anatase to Rutile Phase Transformation. J. Mater. Sci. 2010, 46, 855-874.


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(31) Yue, W.; Randorn, C.; Attidekou, P. S.; Su, Z.; Irvine, J. T. S.; Zhou, W. Syntheses, Li Insertion, and Photoactivity of Mesoporous Crystalline TiO2. Adv. Func. Mater. 2009, 19, 28262833. (32) Dariani, R. S.; Esmaeili, A.; Mortezaali, A.; Dehghanpour, S. Photocatalytic Reaction and Degradation of Methylene Blue on TiO2 Nano-Sized Particles. Optik 2016, 127, 7143-7154. (33) Natarajan, T. S.; Thomas, M.; Natarajan, K.; Bajaj, H. C.; Tayade, R. J. Study on UVLED/TiO2 Process for Degradation of Rhodamine B Dye. Chem. Eng. J. 2011, 169, 126-134. (34) Zheng, J.; Xiong, F.-Q.; Zou, M.; Thomas, T.; Jiang, H.; Tian, Y.; Yang, M. Enhanced Photocatalytic Degradation of Rhodamine B under Visible Light Irradiation on Mesoporous Anatase TiO2 Microspheres by Codoping with W and N. Solid State Sci. 2016, 54, 49-53. (35) Soltani, N.; Saion, E.; Hussein, M. Z.; Erfani, M.; Abedini, A.; Bahmanrokh, G.; Navasery, M.; Vaziri, P. Visible Light-Induced Degradation of Methylene Blue in the Presence of Photocatalytic Zns and Cds Nanoparticles. Int. J. Mol. Sci. 2012, 13, 12242.


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Captions for Tables and Figures Table 1. Physical properties of the mesoporous CdxZn1-xS materials. Table 2. Photocatalytic degradation of organic dyes. Scheme 1. Schematic illustration of photocatalytic degradation of organic dyes. Figure 1. (A) Low- and (B) wide-angle XRD patterns of the mesoporous CdxZn1-xS materials, where x is equal to (a) 1.0 (CdS), (b) 0.9, (c) 0.7, (d) 0.5, (e) 0.3, (f) 0.1 and (g) 0.0 (ZnS). Figure 2. (A) N2-sorption isotherms and (B) BJH pore size distribution curves of the mesoporous CdxZn1-xS materials where x is equal to (a) 1.0 (CdS), (b) 0.9, (c) 0.7, (d) 0.5, (e) 0.3, (f) 0.1 and (g) 0.0 (ZnS). Figure 3. (a – c) SEM and (d – f) TEM images of the mesoporous (a, d) CdS, (b, e) Cd0.5Zn0.5S and (c, f) ZnS. Figure 4. (A) Diffuse reflectance UV-visible spectrums and (B) Kubelka -Munk plots of the mesoporous CdxZn1-xS materials, where x is equal to (a) 1.0 (CdS), (b) 0.9, (c) 0.7, (d) 0.5, (e) 0.3, (f) 0.1 and (g) 0.0 (ZnS). Figure 5. Changes of relative concentration of (A) methylene blue and (B) rhodamine B by using the mesoporous (a) CdS and (b) ZnS. Figure 6. (A, C) Changes of relative concentration and (B, D) linear plots of ln (C/C 0) for the photocatalytic degradation of (A, B) methylene blue and (C, D) rhodamine B, respectively, by using (a) P25 (TiO2), the mesoporous CdxZn1-xS materials where x is equal to (b) 0.9, (c) 0.7, (d) 0.5, (e) 0.7 and (f) 0.1.


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Figure 7. Schematic relative band alignment of the mesoporous CdxZn1-xS and redox potentials of hydrogen and oxygen (vs. NHE). Figure 8. Schematic illustration of relative band alignment of the mesoporous CdxZn1-xS and HOMO-LUMO level of methylene blue (MB) and rhodamine B (RhB). HOMO-LUMO level of organic dyes are calculated by DFT B3LYP/16301G* method.


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Table 1. Physical properties of the mesoporous CdxZn1-xS materials. SBET

Vtotal

Dp

Eg

(m2g-1) a

(cm3g-1) b

(nm) c

(eV) d

CdS

81.2

0.38

18.3

2.43

Cd0.9Zn0.1S

85.1

0.38

21.7

2.47

Cd0.7Zn0.3S

79.8

0.32

18.0

2.61

Cd0.5Zn0.5S

102.9

0.67

19.7

2.78

Cd0.3Zn0.7S

131.8

0.65

20.1

3.10

Cd0.1Zn0.9S

119.8

0.93

19.7

3.47

ZnS

123.7

0.82

19.6

3.64

Material

a

Surface areas were calculated by BET method. b Total pore volumes were estimated from the N2-

sorption isotherms at p/p0 = 0.99. c Pore sizes were calculated from the adsorption branches of the N2-sorption isotherms by BJH method.

d

Band gap energies were estimated from the Kubelka-

Munk plots.


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Table 2. Photocatalytic degradation of organic dyes. kapp (MB)

t1/2 (MB)

kapp (RhB)

t1/2 (RhB)

(min-1) a

(min) b

(min-1) a

(min) b

P25(TiO2)

2.62 × 10-4

2641

5.07 × 10-4

1367

CdS

5.12 × 10-2

13.5

6.01 × 10-2

11.5

Cd0.9Zn0.1S

5.01 × 10-2

13.8

7.12 × 10-2

9.74

Cd0.7Zn0.3S

7.88 × 10-2

8.80

9.53 × 10-2

7.27

Cd0.5Zn0.5S

9.30 × 10-3

74.5

9.57 × 10-3

72.5

Cd0.3Zn0.7S

2.01 × 10-3

345

2.63 × 10-3

264

Cd0.1Zn0.9S

1.19 × 10-3

583

1.44 × 10-3

480

ZnS

9.04 × 10-4

767

9.32 × 10-4

743

Material

a

Apparent rate constant is calculated by first order equation, ln (C/C0) = – kapp·t. b Half-life time

of organic dyes were estimated by equation t1/2 = (ln 2)/kapp.


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Table 3. Photocatalytic degradation of methylene blue and rhodamine B by various nanostructured materials.

Material

Light source

Dye

Catalyst concentration

TiO2 NPs32

9 V UV-A

MB, 10 mg/L

Mesoporous TiO231

250 W Fe-doped halide UV bulb

P-25 TiO233

Degradation Time (min)

Ratio (%)

0.5 g

120

100

MB, 5 × 10-5 M

0.1 g/200 mL

180

96

UV-LED

RhB, 2.08 × 10-5 M

1.6 g/L

180

96

Mesoporous TiO234

300 W Xe lamp

RhB, 1.5 × 10-5 M

0.05 g/50 mL

120

Visible-Light Driven Photocatalytic Degradation of Organic Dyes

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